1. Field of the Invention
This invention relates to a pilot valve and/or proportional valve.
2. Description of the Related Art
Pilot valves and/or proportional valves are known in hydraulics, for example as piston slide valves. A valve slide or piston is moved in a valve housing relative to control edges such that desired flow cross sections for a flow medium that is guided through the valve and that is to be controlled result for various control positions of the valve slide. Another example has the valve designed as a rotary slide valve, such that the different control positions are defined by various rotational positions of the rotary valve relative to control edges in the housing.
Pilot controlled and directly controlled valves differ. With pilot controlled valves, the valve body is hydraulically moved, such that hydraulic valves are used for the generation of the hydraulic pilot forces, which as a rule are smaller and must work faster than the specified main stage. With directly controlled valves, an actuator, for example an electric motor or a solenoid, acts directly on the valve body of the main stage. Directly controlled valves are simpler in design. However, they are limited by the control forces that must be applied, the construction size, and the regulating speed of the valve body.
Independent of the actuating method, it is often demanded from hydraulic valves that, with the elimination of a control signal or when shifting the control circuits into an energy-free state, the valve body be moved into in a constructively defined position. This position is generally referred to as the failsafe position. In an energy-free control circuit, this position is also referred to as the starting position, and should entail a hydraulically defined and manageable, as well as a safe, state.
Traditionally with piston slide valves, the piston slides or valve bodies are acted upon both of their axial ends respectively by spring force in order to thereby push the piston slide mechanically into a central failsafe position if pilot pressures or control signals are turned off. See for example Murrenhoff, H., Basics of Fluid Technology, 2005, Section 1: Hydraulic, Page 227, ISBN 3-8265-946-0. The principle of spring centering, however, has disadvantages. The spring centered position is defined by the forces of equilibrium between the centering springs located at both face ends. Unavoidable friction and hysteresis results in the failsafe position being set only with uncertainty or with variance. Moreover, the spring centering force of both springs act continuously, in other words also during nominal operation of the valve. The operating force of the valve is always directed against the centering force, due to which a comparably higher operating force has to be applied. This again is detrimental in regards to adjustment accuracy, as well as to the costs and size of the valve drive.
What is needed in the art is a pilot valve or proportional valve, having a strongly reproducible failsafe position or starting position and wherein the centering force in nominal operation does not negatively affect the adjustment force.